1. Field of the Invention
The present invention relates to a method and an apparatus which are suitable for use in measuring a spectral irradiance distribution.
2. Description of the Related Art
When a computer, such as a personal computer (PC, hereinafter also called the computer) processes a color image, such as editing a color image captured into the computer using a scanner while checking the color image displayed on the monitor, or printing a color image using a color printer, colors of an object for processing have to be accurately calculated by the computer.
Generally, the basic colors of an object are expressed in the form of tristimulas XYZ values of the light reflected from the object (Gxc3xcnter Wyszecki and W. S. Stiles, xe2x80x9cColor Science: Concepts and Methods, Quantitative Data and Formulae Second Edition, 3.3.8 Calculation of CIE Tristimulus Values and Chromaticity Coordinatesxe2x80x9d, John Wiley and Sons, Inc. (1982)). Accurate calculation of tristimulas XYZ values requires the spectral irradiance distribution of light irradiating the object.
A spectral irradiation distribution is a physical value indispensable for accurate processing of a color image. For example, tristimulas XYZ values are essential factors to estimate a print image to be printed by the printing machine.
The conventional method of measuring a spectral irradiance of the light irradiating a particular surface is exemplified by a method of dividing into spectral distributions by a spectral filter, and measuring the respective spectral irradiance distributions of the individual spectral distributions by a plurality of photosensitive sensors in accordance with various wavelengths of the spectral distributions. This spectral irradiance distribution measurement is performed by a dedicated apparatus.
However, such conventional apparatus has to be equipped with these photosensitive sensors having different optical sensitivities as many as the number of the wavelengths by which the measurement is performed. As a result, the conventional apparatus is complicated and very expensive due to the increased number of the photosensitive sensors.
As spectral irradiance distribution measuring method using photosensitive sensors whose number is less than that of the measuring bands are disclosed in (1) Shoji Tominaga, xe2x80x9cMultichannel Vision System for Estimating Surface and Illumination Functionsxe2x80x9d, J. Opt. Soc. Am. A, Vol. 13, No. 11, 2163-2173 (November 1996), in which a spectral irradiance distribution of the light irradiating an object is measured as shown in FIG. 9, and (2) Japanese Laid-Open (Kokai) Publication No. HEI 11-132853, in which a spectral irradiance distribution of a light source is directly measured as shown in FIG. 10.
These two conventional methods will now be described more with reference to FIGS. 9 and 10.
(1) Method of measuring a spectral irradiance distribution of light irradiating an object:
With this conventional method of FIG. 9, the spectral irradiance distribution of the light irradiates an object 183 (irradiating light), whose spectral reflectance is not given, from a light source 180 and the spectral reflectance are calculated. At that time, the object 183 is photographed by a 6-band camera 182, which has sensitivity to 6 colors of RGB colors (red, green, blue) and to the other three colors, and then the spectral irradiance distribution of the irradiating light and spectral reflectance of the object 183 are measured based on the photographed image.
Since unknown coefficients and numbers upon the measurement are many, special equipment called the 6-band camera 182 is used. Further, the measurable spectral irradiation distribution of irradiating light is limited to such range so as to expressed by a linear combination of three components.
(2) Method of measuring a spectral irradiance distribution of a light source directly:
In this conventional apparatus of FIG. 10, plural optical filters 281 respectively having different spectral transmittances are individually disposed on the irradiating path of a light source 280 emitting light to measure emitting light thereof. The plural sensors 282 respectively installed in accordance with the individual optical filters 281 so as to receive the light transmitted through the optical filters 281. As a result, a spectral irradiance distribution of the light is estimated referring to the spectral transmittances of the optical filters 281.
Subsequently, outputs from the optical sensors 282 are converted into voltage values by an amplifier and then are further converted into digital data by an A/D converter to calculate the spectral irradiance distribution and chromatic characteristic the light source 280 in terms of the digital data on a computer.
However, the above-mentioned conventional methods have the following problems:
One method of FIG. 9 requires special equipment of 6-band camera, and a measurable spectral irradiation distribution of irradiating light is limited to such a range so as to be expressed by a linear combination of three components. Generally, indoor light sources are various types, many of which are complicated. Therefore it is practically difficult that all types of indoor light sources are measured under the same restriction that a spectral irradiation distribution is measured within a range such as to be expressed by a linear combination of three components.
In the other method of FIG. 10, the spectral irradiance distribution of the light source 280 is directly measured. It is impossible to measure the spectral irradiance distribution of the light irradiating an object surface. And there is a significant restriction that a spectral irradiance distribution of only light sources very similar in spectral irradiance distribution to a single predetermined light source.
Accordingly, with either these two conventional methods, it is difficult to measure a spectral irradiance distribution of light irradiating an object surface without using any expensive measuring equipment.
With the foregoing problems in view, it is an object of the present invention to provide a method of measuring a spectral irradiance distribution of light irradiating an object surface without using expensive measuring equipment. Another object of the invention is to provide an apparatus for carrying out the above-mentioned method.
In order to attain the above first-named object, according to a first generic feature of the present invention, there is provided a method of measuring a spectral irradiance distribution of light, comprising the steps of: receiving the light on s (natural number) optical sensors, which respectively have s kinds of given spectral sensitivities, via n (natural number) light-transit sections, which respectively have n kinds of given optical characteristic coefficients; detecting individual responses for the light (receiving irradiances of the light), which is received via the n light-transit sections, by each and every one of the s optical sensors, and obtaining (nxc3x97s) responses for the received light; and calculating m (natural number) spectral irradiances for m kinds of wavelengths as a spectral irradiance distribution of the light, based on one or more linear formulae established between the optical characteristic coefficients, the spectral sensitivities, the (nxc3x97s) receiving irradiances, and the spectral irradiance distribution of the light. With this method, it is possible to calculate a spectral irradiance distribution of light with ease and to make the apparatus simple in construction with the reduced number of the optical sensors.
As a preferable feature, the optical sensors and the light-transit sections are arranged in such a manner that the number (nxc3x97s) of the responses for the light detected by the optical sensors is equal to or larger than the number (m) of the spectral irradiances to be calculated based on the linear formulae; and the m spectral irradiances are calculated by directly solving the linear formulae. With this preferable feature, it is possible to solve the one or more linear formulae improving the reliability of the solutions of the linear formulae.
As another preferable feature, the optical sensors and the light-transit sections are arranged in such a manner that the number (nxc3x97s) of the responses for the light detected by the optical sensors is smaller than the number (m) of the spectral irradiances to be calculated based on the linear formulae; and the m spectral irradiances are calculated by solving the linear formulae under a predetermined constraint. With this preferable feature, it is possible to make the apparatus simple in construction with the reduced number of the optical sensors and to improve the reliability of the solutions of the linear formulae.
As still another preferable feature, the predetermined constraint is such that the spectral irradiance distribution of the light is a positive value, or is expressed by a linear combination of predetermined spectral irradiance distributions. With this preferable feature, it is possible to solve the linear formulae with ease and to improve the reliability of the solution of the linear formulae.
As a further preferable feature, another constraint is added to the predetermined constraint, the another constraint being such that: the spectral irradiance distribution of the light is a positive value; and the spectral irradiance distribution of the light is expressed by linearly combining the predetermined spectral irradiance distribution with a non-negative coefficient. With this preferable feature, it is possible to improve the reliability of the solutions the linear formulae.
As a still further preferable feature, the predetermined spectral irradiance distribution is a principal component of estimated spectral irradiance distributions of light sources, or is a linear combination with principal components of estimated spectral irradiance distributions of light sources, or is an estimated spectral irradiance of a light source. With this preferable feature, it is possible to improve the reliability of the solutions the linear formulae.
As an additional preferable feature, the method further comprises: preparing a plurality of sets of the predetermined spectral irradiance distributions; calculating the spectral irradiance distributions of the light respectively using the prepared plural sets of spectral irradiance distributions; and selecting, among a plurality of the calculated spectral irradiance distributions of light, one spectral irradiance distribution whose error in the linear formulae is minimal. With this preferable feature, it is possible to obtain a reliable spectral irradiance distribution.
As still another preferable feature, the light-transit selection is a reflector which reflects the light; the optical characteristic coefficient is a spectral reflectance of the reflector; and the receiving irradiance of the light reflected from the reflector is detected by the optical sensors. With this preferable feature, it is possible to make the apparatus simple in construction.
As a still further preferable feature, the light-transit selection is a filter which allows the light to transmit therethrough; the optical characteristic coefficient is a spectral transmittance of the filter; and the receiving irradiance of the light transmitted through the filter is detected by the optical sensors. With this preferable feature, it is possible to make the apparatus simple in construction.
In order to attain the above second-named object, according to a second generic feature of the present invention, there is provided an apparatus for measuring a spectral irradiance distribution of light, comprising: n (natural number) light-transit sections to which the light is to be irradiated, the n light-transit sections respectively having n kinds of given optical characteristic coefficients; s (natural number) optical sensors for detecting individual responses for the light received via the n light-transit sections, the s optical sensors respectively having s kinds of given spectral sensitivities; and a calculating section for calculating m (natural number) spectral irradiances for each and every one of m kinds of wavelengths as a spectral irradiance distribution of the light based on one or more linear formulae established between the optical characteristic coefficients, the spectral sensitivities, (nxc3x97s) receiving irradiances obtained by the s optical sensors, and the spectral irradiance distribution. With this apparatus, it is possible to calculate a spectral irradiance distribution of light with ease and to make the apparatus simple in construction with the reduced number of the optical sensors.
As a preferable feature, the optical sensors and the light-transit sections are arranged in such a manner that the number (nxc3x97s) of the responses for the light detected by the optical sensors is equal to or larger than the number (m) of the spectral irradiances to be calculated based on the linear formulae; and the m spectral irradiances are calculated by directly solving the linear formulae. With this preferable feature, it is possible to solve the one or more linear formulae improving the reliability of the solutions of the linear formulae.
As another preferable feature, the optical sensors and the light-transit sections are arranged in such a manner that the number (nxc3x97s) of the responses for the light detected by the optical sensors is smaller than the number (m) of the spectral irradiances to be calculated based on the linear formulae; and the m spectral irradiances are calculated by solving the linear formulae under a predetermined constraint. With this preferable feature, it is possible to make the apparatus simple in construction with the reduced number of the optical sensors and to improve the reliability of the solutions of the linear formulae.
As still another preferable feature, the predetermined constraint is such that the spectral irradiance distribution of the light is a positive value, or is expressed by a linear combination of predetermined spectral irradiance distributions. With this preferable feature, it is possible to solve the linear formulae with ease and to improve the reliability of the solution of the linear formulae.
As a further preferable feature, another constraint is added to the predetermined constraint, the another constraint being such that: the spectral irradiance distribution of the light is a positive value; and the spectral irradiance distribution of the light is expressed by linearly combining the predetermined spectral irradiance distribution with a non-negative coefficient. With this preferable feature, it is possible to improve the reliability of the solutions the linear formulae.
As a still further preferable feature, the predetermined spectral irradiance distribution is a principal component of estimated spectral irradiance distributions of light sources, or is a linear combination with principal components of estimated spectral irradiance distributions of light sources, or is an estimated spectral irradiance of a light source. With this preferable feature, it is possible to improve the reliability of the solutions the linear formulae.
As an additional further preferable feature, the apparatus further comprises: preparing a plurality of sets of the predetermined spectral irradiance distributions; calculating the spectral irradiance distributions of the light respectively using the prepared plural sets of spectral irradiance distributions; and selecting, among a plurality of the calculated spectral irradiance distributions of light, one spectral irradiance distribution whose error in the linear formulae is minimal. With this preferable feature, it is possible to obtain a reliable spectral irradiance distribution.
As still another preferable feature, the light-transit selection is a reflector which reflects the light; the optical characteristic coefficient is a spectral reflectance of the reflector; and the receiving irradiance of the light reflected from the reflector is detected by the optical sensors. With this preferable feature, it is possible to make the apparatus simple in construction.
As a still further preferable feature, the light-transit selection is a filter which allows the light to transmit therethrough; the optical characteristic coefficient is a spectral transmittance of the filter; and the receiving irradiance of the light transmitted through the filter is detected by the optical sensors. With this preferable feature, it is possible to make the apparatus simple in construction.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.